1. Field of the Invention
The present invention generally relates to a digital-to-analog converter (DAC), in particular, to a source driver and a DAC thereof which produce no gamma coupling.
2. Description of Related Art
FIG. 1 is a diagram of a conventional source driver. Referring to FIG. 1, the conventional source driver 110 includes a shift register 111, a data latch 112, a line latch 113, and a digital-to-analog converter 114. The shift register 111 generates a plurality of shift signals having different delay times according to a start pulse EIO output by a timing controller 120. Next, the data latch 112 latches a digital data DATA output by the timing controller 120 according to the shift signals and transmits the result to the line latch 113. After that, the line latch 113 latches the output of the data latch 112, namely, the line latch 113 updates the output of its own according to a line latch signal STB output by the timing controller 120. Finally, the DAC 114 converts the output of the line latch 113 into an analog voltage according to a gamma voltage Vgamma output by a gamma voltage generator 130, so that a display panel 140 can display an image according to the analog voltage.
The circuit of the DAC 114 may adopt a ROM-type structure or a binary structure. FIG. 2 is a diagram of a conventional ROM-type DAC. Referring to FIG. 2, a gamma voltage generator 210, a line latch 220, and a DAC 230 are illustrated, and VDD and GND respectively represent a supply voltage and a common voltage. The DAC 230 includes a reference voltage generation unit 240 and a selection unit 250, wherein the selection unit 250 further includes a decoding unit 251 and a plurality of selection switches 252. In the present embodiment, the selection switches 252 are all implemented with metal oxide semiconductor (MOS) transistors. The DAC 230 is referred as a ROM-type DAC because that the couplings between the selection switches 252 preset the selection unit 250 as a ROM-type selection unit.
The decoding unit 251 decodes an input data DIN output by the line latch 220 so as to obtain the control signals DP0, DP0B, DP1, DP1B, DP2, and DP2B for controlling the on/off the selection switches 252, wherein the control signals DP0B, DP1B, and DP2B are respectively the reverse signals of the control signals DP0, DP1, and DP2, and each of the selection switches 252 receives the control signals as illustrated in FIG. 2. The on/off of all the selection switches are determined by the control signals. Eventually, the selection unit 250 selects one of a plurality of voltage levels output by the reference voltage generation unit 240 according to the input data DIN, and the selected voltage level is served as an analog voltage AOUT which is output correspondingly by the DAC 230.
However, during a data conversion period of the input data DIN, a transient gamma coupling (also referred as gamma short) may be produced in such a DAC structure.
FIG. 3 is a diagram of a conventional binary DAC. Referring to FIG. 3, a gamma voltage generator 210, a line latch 220, and a DAC 430 are illustrated, and VDD and GND respectively represent a supply voltage and a common voltage. The DAC 430 includes a reference voltage generation unit 240 and a selection unit 450, wherein the selection unit 450 further includes a decoding unit 251 and a plurality of selection switches 452. The DAC 430 is referred as a binary DAC because that the couplings between the selection switches 452 preset the selection unit 450 as a binary selection unit.
Referring to both FIG. 2 and FIG. 3, it can be understood by comparing the two selection units that as to a resistor at the same position in the reference voltage generation unit, the selection switches coupled to the resistor in the binary DAC is less than the selection switches coupled to the resistor in the ROM-type DAC. Thereby, the possibility of gamma coupling produced by the binary DAC is greater than the possibility of gamma coupling produced by the ROM-type DAC. In other words, the binary DAC produces more gamma couplings than the ROM-type DAC.